defmodule AOC.Day9 do
  @preamble_len 25

  def find_invalid(queue, [hd | tl]) do
    sums =
      :queue.to_list(queue)
      |> Combination.combine(2)
      |> Enum.map(&Enum.sum/1)

    queue =
      :queue.in(hd, queue)
      |> :queue.drop()

    if hd in sums do
      find_invalid(queue, tl)
    else
      hd
    end
  end

  def find_weakness(invalid, [_ | tl] = nums) do
    result =
      Enum.reduce_while(nums, {0, []}, fn num, {sum, l} ->
        sum = sum + num
        if sum < invalid, do: {:cont, {sum, [num | l]}}, else: {:halt, {sum, l}}
      end)

    case result do
      {sum, l} when sum == invalid -> Enum.min_max(l)
      _ -> find_weakness(invalid, tl)
    end
  end

  def parts do
    input = AOC.Util.input_integers(9, 1)
    {preamble, nums} = Enum.split(input, @preamble_len)

    invalid =
      preamble
      |> :queue.from_list()
      |> find_invalid(nums)

    {min, max} = find_weakness(invalid, input)
    {invalid, min + max}
  end
end